The invention relates to a hydraulic piston machine having a piston arranged to move back and forth in a cylinder body.
In such a case, several cylinders containing the pistons are normally arranged in the cylinder body. Hydraulic machines which convert pressure and movement into one another by means of pistons moving back and forth in cylinders have been known for a long time. Such machines work as motors when the pressure of the hydraulic fluid is to be converted into movement. They work as pumps when a movement is to be converted into an increase in pressure of the hydraulic fluid. In machines of that kind the sliding contact surfaces in the cylinders in which the pistons move back and forth suffer extreme stress caused by high sliding speeds and huge bearing pressures. In order to keep wear and tear in such machines as low as possible, it is customary not to let the pistons run directly in the bores which form the cylinders in the cylinder body, but to insert cylinders or bushings in the cylinder body in which the pistons are able to move. In this manner it is indeed possible in many cases to use suitable material pairings between piston and cylinder bushing without having to make the entire cylinder body of the generally more expensive and less stable material of the bushing. It is relatively difficult, however, to secure such bushings in the cylinder body with the required reliability. An adhesive join is in many cases not capable of accommodating the relatively large shear forces between bushing and cylinder body. For that reason, (DE-AS 12 67 985) proposes securing the cylinder bushing by means of a press fit in the cylinder bore in the cylinder body, and causing an enlargement initially projecting inwardly into the cylinder bushing to be reshaped outwardly by means of a mandrel into a corresponding annular groove in the cylinder bore. It is a prerequisite here that the material of the cylinder bushing allows such a reshaping, without the cylinder bushing being damaged. This is normally the case only with metals.
In conjunction with the use of hydraulic fluids which have only slight or even no lubricating properties, it is now wished to provide the material pairing at the sliding contact surface at least on one side with a plastics material. Such plastics materials can be shaped in the manner described only with difficulty. Fixing in a cylinder body is possible, but complicated (see the two prior German patent applications P 43 01 124 and P 43 01 126).
From DE-Z "Der Zuliefermarkt" (The Supply Market), April 1986, A. Knittel, Rotary Metering Pistons of Plastics Material, is it known to manufacture a piston of plastics material, for example, in the form of an injection-moulded part. Such a plastics material piston is unsuitable, however, for use in a hydraulic machine. Most plastics materials are quite unable to withstand the pressures occurring in the cylinder of a hydraulic machine. The pressures lead to deformation of the piston so that this jams in the cylinder bore.